The present invention generally relates to semiconductor devices, and more particularly to a tunable laser diode that has a branched optical cavity for realizing a large shift of laser oscillation.
In the optical telecommunication systems that use the wavelength multiplexing technique, a tunable laser diode that can change the wavelength of the output optical beam for a wide wavelength range is indispensable. Such a tunable laser diode is used for example for an optical local oscillator of optical heterodyne detectors. In the optical heterodyne detectors, a non-linear mixing of two optical beams is achieved at a photodetector, wherein one of the optical beams carries information while the other is a local optical beam that is produced by the optical local oscillator. Thereby, the information carried on the optical beam is converted to an intermediate electrical signal having an intermediate frequency. It should be noted that the intermediate electrical signal contains the information content that has been modulated on the optical beam by any of amplitude modulation, frequency modulation or phase modulation. By changing the wavelength of the optical local oscillator in accordance with the wavelength of the incident optical beam, one can obtain the intermediate frequency signal with a substantially constant frequency. It should be noted that such an optical heterodyne detection is particularly suitable for extracting a desired signal from a number of signals that are multiplexed on the optical beam. In order to realize the optical local oscillator, it will be understood that the development of the tunable laser diode that has the capability of extensive wavelength tuning is essential.
Conventionally, a so-called DBR laser diode is proposed for a tunable laser diode (Kotaki et al., Electronics Letters, Vol. 24, No. 8, 1988, 503-505; Broberg et al., Applied Physics Letters, Vol. 52, No. 16, 1988, pp. 1285-1287). In the DBR laser diode, a Bragg reflector consisting of a corrugation is provided in the optical cavity of the laser diode adjacent to, and in alignment with the active layer so that the corrugation causes a Bragg reflection of the optical beam. There, the optical beam is amplified by the stimulated emission in the active layer as it is reflected back and forth by the corrugation. The shift of the wavelength of the optical beam is achieved by injecting carriers into the Bragg reflector. It should be noted that such an injection of the carries induces a change of the refractive index in the material that forms the corrugation by the plasma effect, and such a change of the refractive index in turn causes a change of the effective pitch of the corrugation. Thereby, the wavelength of the optical beam that establishes the Bragg reflection is changed.
Unfortunately, the magnitude of the wavelength change that is achieved by the DBR laser diode of this prior art is relatively limited. For example, Kotaki et al. op. cit. describes a change of the laser oscillation of only 6.2 nm in the 1.53 .mu.m band, while Broberg et al. op. cit. describes a change of 11.6 nm in the 1.55 .mu.m band. The reason of this unsatisfactory result is attributed to the very fundamental principle of the wavelength shifting that the laser diode of this prior art relies upon. For example, in the case of the DBR laser diode reported by Broberg et al. op. cit., the laser oscillation may be interrupted when the injection of the carriers into the corrugation is reduced for changing the refractive index of the Bragg reflector. Since the active layer extends to the region of the Bragg reflector, the reduction of the carrier injection to the Bragg reflector inevitably results in the reduction of the optical gain. The attempt to compensate for such a decreased carrier injection by increasing the carrier injection in the active layer is generally limited because of the problem of excessive heating and hence the reliability of operation of the laser diode. Once the laser oscillation is established, on the other hand, there is a tendency for the carrier density to be clamped at a constant level. This effect also acts to reduce the range of the wavelength tuning. In the DBR laser diode reported by Kotaki et al., op. cit., the tuning range is determined by the maximum injection current to the DBR region, which in turn is limited by the heating effect.
As a tunable laser diode, a DFB laser diode having segmented electrodes has also been proposed (Kotaki et al., Electronic Letters, Vol. 25, No. 15, 1989, pp. 990-992). This prior art laser diode has an active layer extending throughout the optical cavity, and there is provided a corrugation in the optical cavity extending from a first end to a second, opposite end of the laser diode, as is usual in the DFB laser diode. In the corrugation, there is provided a .lambda./4-shift point where the phase of the corrugation is shifted by a quarter (1/4) of the pitch or wavelength of the corrugation. Thereby, there occurs a strong concentration of optical radiation in the optical cavity in correspondence to the .lambda./4-shift point. This in turn causes a strong depletion of carriers in correspondence to the .lambda./4-shift point due to the facilitated stimulated emission.
In this prior art DFB laser diode, the electrodes that form the segmented electrodes are provided in alignment with the optical axis with a physical separation from each other, wherein one of the electrodes is provided in correspondence to this .lambda./4-shift point. By controlling the injection current to the electrode that is located immediately above the .lambda./4-shift point and further by controlling the injection current to the rest of the electrodes independently, the profile of the carrier distribution and hence the intensity distribution of the optical radiation is modified as desired. For example, by decreasing the carrier injection to the electrodes that are offset from the .lambda./4-shift point, the non-uniform distribution of the carriers in the optical cavity is enhanced. Thereby, the overall refractive index of the optical cavity is decreased by the plasma effect, which in turn results in a decreased Bragg wavelength. Such a decrease of the Bragg wavelength results in a decrease of the oscillation wavelength of the laser diode.
When increasing the oscillation wavelength, on the other hand, the injection current at the electrode above the .lambda./4-shift point is increased such that carrier distribution in the optical cavity becomes more uniform. With such a change of the carrier distribution profile, the refractive index of the optical cavity is increased as a whole, which in turn results in an increase of effective cavity length of the laser diode. Thereby, the oscillation wavelength of the laser diode increases.
This prior art device also has a drawback in that the magnitude of the wavelength shift is not sufficient for the optical local oscillator mentioned previously. This problem becomes particularly conspicuous when increasing the oscillation wavelength. As already noted, the increase of the oscillation wavelength is achieved by decreasing the carrier injection to the electrodes located at opposite sides of the .lambda./4-shift point. However, the magnitude of the decrease of the carrier injection is limited by the constraint that the laser oscillation has to be sustained. Further, the increase of the carrier injection to the .lambda./4-shift point is limited, as an excessive increase of the carrier injection tends to cause a decrease of the oscillation wavelength by the adversary plasma effect, which acts oppositely to the desired effect. Generally, the DFB laser diode of this type provides a range of wavelength shift that is even smaller than that of the first type device mentioned previously. For example, Kotaki et al. op. cit. reports a wavelength shift of 2.2 nm.
In order to realize a much larger shift of oscillation wavelength, a third type tunable laser diode that uses a split optical cavity has been proposed (Schilling et. al., Electronics Letters, Vol. 26, No. 4, 1990, pp. 243-244; Schilling et al., IEEE J. Quantum Electronics, Vol. 27, No. 6, 1991, 1616-1624; Hildebrand et al., 17th ECOC'91/IOOC'91, 1991, Paper #Tu.A5.1; Idler et. al., Electronics Letters, Vol. 27, No. 24, 1991, pp. 2268-2270). In this type of tunable laser diode, there is provided a Y-shaped, branched optical cavity that divides the optical beam into two beams. The two optical beams thus produced cause an interference in correspondence to the part where the two branches merge with each other. By controlling the refractive index of one or both of the branches so that there occurs a constructive interference between the two optical beams, one can achieve a laser oscillation at a desired oscillation wavelength.
Next, the principle of this type of tunable laser diode will be explained in more detail with reference to FIG. 1, which shows the structure of a conventional tunable laser diode 10.
Referring to FIG. 1 showing the laser diode 10 in a plan view, the device includes two optical cavities B.sub.1 and B.sub.2 that merge with each other in correspondence to a gain region 10a. In other words, the gain region 10a is common to the optical cavities B.sub.1 and B.sub.2. In correspondence to the gain region 10a, there is provided an active part of the laser diode that amplifies the optical beam passing therethrough by stimulated emission. Further, in correspondence to the part where the cavities B.sub.1 and B.sub.2 are branched from each other, regions 10b and 10c are formed respectively for modifying the refractive index thereof. The optical cavities B.sub.1 and B.sub.2 have respective optical lengths L.sub.1 and L.sub.2, wherein the optical length L.sub.1 of the cavity B.sub.1 is set different from the optical length L.sub.2 of the cavity B.sub.2. In the illustrated example, the optical length L.sub.2 is set larger than the optical length L.sub.1.
FIGS. 2(A) and 2(B) show the standing waves that are formed in the optical cavities B.sub.1 and B.sub.2 by the optical beam produced by the gain region 10a. There, it will be noted that the phase of the optical beam in the optical cavity B.sub.1 and the phase of the optical beam in the optical cavity B.sub.2 coincide with each other, indicating that there is established a constructive interference of the two optical beams in the gain region 10a. In other words, FIGS. 2(A) and 2(B) show the case wherein the laser diode produces a strong coherent optical beam.
FIG. 3 shows various longitudinal modes of laser oscillation that correspond to various standing waves formed in an optical cavity. As is well known in the art, a laser diode having an optical cavity oscillates at discrete wavelengths in correspondence to the standing waves that are established in the optical cavity. Thereby, each mode is separated from the adjacent mode by a frequency .DELTA..nu. that is given as EQU .DELTA..nu.=c/2nL, (1)
where c represents the speed of light in the vacuum, n represents the refractive index of the medium that forms the optical cavity, and L represents the axial length of the optical cavity. The foregoing relationship can be rewritten in terms of the wavelength .lambda. of the optical beam such that: EQU .DELTA..lambda.=.lambda..sup.2 /2nL, (2)
where .DELTA..lambda. represents the wavelength separation between the adjacent modes. Eq. (2) indicates that the wavelength separation .DELTA..lambda. is determined by the wavelength .lambda. of the optical beam, the refractive index n and the length L of the optical cavity. It should be noted that the refractive index n is included in the denominator of Eq. (2).
On the other hand, the oscillation wavelength of each longitudinal mode is given as: EQU .lambda..sub.m =2nL/M (3)
where m represents the order of the mode.
Eq. (3) indicates that the wavelength .lambda..sub.m is proportional to the refractive index n of the optical cavity. In other words, the wavelength .lambda..sub.m changes linearly with the change of the refractive index n while maintaining a generally constant wavelength separation .DELTA..lambda. from the adjacent modes. This feature will he noted in the explanation given below concerning the interference of two optical beams in the branched optical cavity.
Referring to FIG. 3 again, a curve g represents the gain spectrum of the laser diode. Further, FIG. 3 shows also a cavity loss for each mode. Thus, it will be understood that each longitudinal mode has an optical gain and a cavity loss that are pertinent thereto. When the laser diode is biased to a level below the oscillation threshold, each optical mode has an optical gain that is proportional to the gain spectrum g. With increasing injection current, the optical gain increases. Thus, once the gain of one mode has exceeded the cavity loss, the laser oscillation starts at this mode. There, the gain spectrum is fixed at the state where the laser oscillation started first, and the optical amplification for the other mode is suppressed. Thus, the laser oscillation occurs selectively at the mode that initially started the oscillation, even when the injection of the carriers is increased thereafter.
Next, the interference of two optical beams produced in the laser diode of FIG. 1 in correspondence to the optical cavities B.sub.1 and B.sub.2 respectively will be examined with reference to FIGS. 4 and 5.
Referring to FIG. 4, the spectrum of the first optical cavity B.sub.1 includes the modes m1, m1.+-.1, m1.+-.2, . . . , and the spectrum is superposed on the spectrum of the second optical cavity B.sub.2 that includes the modes m2, m2.+-.1, m2.+-.2, . . . There, each mode of the first optical cavity B.sub.1 is separated from each other such mode by a wavelength separation .DELTA..lambda..sub.1, while each mode of the second optical cavity B.sub.2 is separated from each other such mode by a wavelength separation .DELTA..lambda..sub.2. It should be noted that the wavelength of the m1-th mode of the first optical cavity B.sub.1 and the wavelength of the m2-th mode of the second optical cavity B.sub.2 coincide with each other at a wavelength .lambda..sub.0 (.lambda..sub.m1 =.lambda..sup.m2 =.lambda..sub.0). Further, in correspondence to Eq. (2), the wavelength separation between the adjacent modes in the first optical cavity B.sub.1 is represented as EQU .DELTA..lambda..sub.1 =.lambda..sub.0.sup.2 /2n.sub.1 L.sub.1
where n.sub.1 and L.sub.1 represent respectively the refractive index and the effective length of the optical cavity B.sub.1, while the wavelength separation in the second optical cavity B.sub.2 is represented as: EQU .DELTA..lambda..sub.2 =.lambda..sub.0.sup.2 /2n.sub.2 L.sub.2
where n.sub.2 and L.sub.2 represent respectively the refractive index and the effective length of the optical cavity B.sub.2.
FIG. 5 shows the wavelength of the various modes formed in the first and second optical cavities B.sub.1 and B.sub.2 of the tunable laser diode of FIG. 1 while changing the refractive index n.sub.2 of the optical cavity B.sub.2 with respect to the refractive index n.sub.1 of the optical cavity B.sub.1. There, the refractive index n.sub.1 is held constant. It should be noted that the relationships of FIG. 5 are obtained for the tunable laser diode that has a length L.sub.1 of 343 .mu.m for the optical cavity B.sub.1 and a length L.sub.2 of 347 .mu.m for the optical cavity B.sub.2, with the length of the part 10a set to 200 .mu.m, the length of the part 10b set to 143 .mu.m, the length of the part 10c set to 147 .mu.m.
Referring to FIG. 5, it will be noted that the wavelength .lambda..sub.2m.+-.i of the mode 2m.+-.i (i=1, 2, 3 . . . ) changes linearly with the change of the refractive index n.sub.2 represented as .DELTA.n.sub.2. On the other hand, the wavelength .lambda..sub.1m.+-.i of the mode 1m.+-.i (i=1, 2, 3, . . . ) does not change as represented by the vertical lines. Further, it should he noted that the wavelength separation .DELTA..lambda..sub.2 in the cavity B.sub.2 is set slightly smaller than the wavelength separation .DELTA..lambda..sub.1 in the cavity B.sub.1 (.DELTA..lambda..sub.1 -.DELTA..lambda..sub.2 =0.01 nm). Thereby, there appear a number of intersections as represented by the solid circles wherein the phase of the optical beam in the optical cavity B.sub.1 coincides with the phase of the optical beam in the optical cavity B.sub.2. In other words, the solid circles represent the wavelengths of the optical beam that the tunable laser diode of FIG. 1 produces. By changing the refractive index n.sub.2, the actual oscillation wavelength of the laser diode changes along the lines such as a line C shown in FIG. 5 that connects the solid circles. There, the line C connects the solid circles E, M, O and J, wherein the solid circle E corresponds to the wavelength .lambda..sub.0. In FIG. 5, it should be noted that there are actually one hundred .lambda..sub.m1 modes included between the wavelength of 1.55 .mu.m that corresponds to .lambda..sub.0 and the wavelength of 1.65 .mu.m, and between the wavelength of 1.45 .mu.m to the wavelength of 1.55 .mu.m. The illustration of all these modes is not attempted, as such an illustration would excessively complicate the drawing.
The relationship of FIG. 5 indicates that one can achieve a change of the wavelength of the optical beam produced by the laser diode of FIG. 1 of as much as 100 .mu.m by merely changing the refractive index n.sub.2 by about 0.15%. It should be noted that a change of the refractive index of this magnitude is caused in response to a very small change of the wavelength .lambda..sub.2m.+-.i, of only 0.99 nm, in the second optical cavity B.sub.2. By combining with the first optical cavity B.sub.1 and by using the interference of the optical beams in the first and second cavities B.sub.1 and B.sub.2, the range of the wavelength shift is significantly expanded.
In FIG. 5, it will be noted that there exist a plurality of oscillation modes for each given refractive index n.sub.2. For example, when the refractive index change .DELTA.n.sub.2 is zero, the laser oscillation can occur at the wavelengths corresponding to the points A, E and I. When the parameter .DELTA.n.sub.2 is set to 0.15%, the laser oscillation can occur at the points B, F and J. In the actual device of FIG. 1, the laser oscillation occurs only at one point for a given .DELTA.n.sub.2, because of the gain spectrum as will be described below.
FIG. 6 shows a typical gain spectrum of the laser diode of FIG. 1. It should be noted that the gain spectrum itself is related to the material that forms the active layer of the laser diode, not to the structure of the optical cavity.
Referring to FIG. 6 again, it will be noted that the oscillation can occur at any of the points A, E and I when the parameter .DELTA.n.sub.2 is set to zero as already mentioned. On the other hand, the gain spectrum (a) of FIG. 6 indicates that the points A and E have a gain that is smaller than the gain at the point I. Thus, the laser oscillation occurs actually at the single point I, with the wavelength of 1.65 .mu.m. When the injection current is set in correspondence to the gain spectrum (d), the laser oscillation occurs preferentially at the point E where the optical gain is the largest. In other words, as a result of the combination with the gain spectrum of FIG. 6, the laser diode of FIG. 1 having the characteristic of FIG. 5 operates substantially as a single mode tunable laser diode.
In this conventional laser diode, it should be noted that a wavelength change that exceeds 100 nm cannot be achieved. For example, when the parameter .DELTA.n.sub.2 is increased from zero, the oscillation wavelength of the laser diode increases along the line E-J of FIG. 5 until it reaches a wavelength value corresponding to the point N. Here, the gain spectrum (d) of FIG. 6 is assumed. When the wavelength has exceeded the point N and reached the point O, it will be understood from the gain spectrum (d) of FIG. 6 that the gain of the point P located at the shorter wavelength side of the point E exceeds the gain of the point O. There, the oscillation wavelength jumps from the point O to the point P. Thereby, the wavelength decreases by about 100 nm. In other words, the wavelength change that is achieved by the device of FIG. 1 is limited in the range changes between the point P and point M and cannot exceed 100 nm even when the refractive index n.sub.2 of the second optical cavity B.sub.2 is changed by 0.15% or more. It will be noted from FIG. 5 that a similar jump of the oscillation wavelength would be repeated between other lines such as the line A-K and the line B-L.